I can't choose between Rarity and Fluttershy (the order I chose to type those names even is conflicted). I adore them both so much. I relate to Fluttershy better, but Rarity is absolutely awesome! Argh, why must life be full of THESE choices? These are the ones that really shape who we are.
Not to say I'm not enamored with all of them, including Derpy Hooves! D:
However if I based it on solely who I would be most like (yeah, in MLP:FiM, I can't even believe I'm considering this) it would have to be Twilight Sparkle. She however isn't my favourite based on her parts in the show, shows based on her, songs etc...
I'd like to take this time to say, I do have other more substantial things going on in my life. Honest...
Edit: I broke and settled on Fluttershy, and I knew I'd regret it because Rarity deserves more love than she gets from the community =(